The present invention relates to an apparatus for controlling speed ratios of a continuously variable transmission (hereinafter referred to as CVT) installed in a vehicle, and particularly to an apparatus for controlling a continuously variable transmission to obtain a fixed vehicle speed when an engine of the vehicle is acted as a brake.
A CVT with a V-belt extending between driving and driven pulleys, with one or both of the pulleys being able to adjust the size of their respective V-shaped openings which hold the V-belt, thereby controlling the revolution ratio between the driving and driven pulleys. The size of the V-shaped opening is varied by changing the amount of pressure being fed into or drained from a hydraulic cylinder of either of the pulleys. Hence, the effective diameter of the pulley can be varied by changing the amount of pressure being fed into or drained from the hydraulic cylinder, thereby allowing for a continuously varying ratio of driving pulley diameter to driven pulley diameter. The V-shaped openings on the driving and driven pulleys are each defined by an area between a fixed member of the pulley and a movable member of the pulley, the size of the V-shaped opening being dependent upon the pressure inputted to or drained from the hydraulic cylinders. In order to provide hydraulic pressure to each of the movable pulleys, a hydraulic apparatus, is provided. Such a CVT has been employed from the standpoint that it can obtain a minimum fuel consumption rate.
In a vehicle installed with a CVT which transmits an engine power to wheels with a continuous transmission ratio, a minimum fuel consumption rate, which is varied according to an engine load, can be obtained by controlling a speed ratio of the rotation speed of an output shaft to the rotation speed of an input shaft (=(the rotation number of the output shaft)/(the rotation number of the input shaft)). According to a prior CVT, when a vehicle runs on the condition when the engine is acted as a brake and the engine brake is effected, the following operation is made: To minimize the fuel consumption, an operation is made such as the speed ratio is set to its maximum value and the effect of the engine brake is reduced. According to another operation, the connection between the engine and the CVT is disengaged by an actuation of a clutch to minimize the fuel consumption. Hence, when a vehicle runs on the condition with the effect of the engine brake, for example, on a condition that the vehicle goes down a slope, the vehicle speed increases. To obtain the appropriate vehicle speed, the frequent braking operation is necessitated.